In many integrated circuits (ICs), a power-on reset (POR) circuit is provided in association with the main circuitry of the IC. The POR circuit ensures that the main circuitry starts functioning in a consistent and known state upon being powered on. When power is applied to a system in which the IC resides, the supply voltage will rise to a normal operating level that exceeds a level sufficient for operation of the main circuitry. However, as the supply voltage rises to the normal operating level, the main circuitry cannot function properly, and even after the supply voltage reaches the normal operating level, the main circuitry is not guaranteed to start functioning in a consistent and known state.
As the supply voltage rises after power is applied, the POR circuit detects when the supply voltage exceeds a POR threshold voltage, which corresponds to an acceptable operating level for the main circuitry and provides a POR signal. The POR signal is used to reset all or part of the main circuitry and/or trigger a function provided by the main circuitry once the supply voltage has reached the acceptable operating level. The acceptable operating level is generally lower than the normal operating level, and as such, the POR signal may be provided as the supply voltage continues to rise above the acceptable operating level to the normal operating level. Thus, the POR circuit ensures that the supply voltage has reached an acceptable operating level for the main circuitry prior to providing the POR signal to reset the main circuitry and/or trigger certain functions to ensure proper operation of the main circuitry after being powered on.
Energy efficiency is a top priority for many applications. Once a system is powered on, the system cycles between on and standby states. In the on state, the system provides normal functions and draws more current. In the standby state, the system goes into a very low current mode with significantly reduced functionality. For many ICs, the POR circuit is a dominant contributor to current draw in standby mode. Unfortunately, reducing the current draw of the existing POR circuits tends to increase the variability of the POR threshold voltage from one POR circuit to another due to process and temperature variations.
If the POR threshold voltage is too low, reset functions or other functions may not be properly activated because the supply voltage may not reach the level needed to operate the main circuitry. This may result in an operational malfunction or excessive current drain while the supply voltage is rising. Typically, if the rise time of the supply voltage is fast enough, then even if the POR threshold voltage is low, the main circuitry will operate properly because the supply voltage rises to acceptable levels quickly after the POR signal is provided. However, for those systems that have relatively slow rise times on the supply voltage, sporadic failures can occur when the POR threshold voltage is too low. Thus, a fast rise time of the supply voltage cannot be relied upon to compensate for a POR threshold voltage that can vary to excessively low levels. Further, the POR threshold voltage typically may not be set higher to avoid potential issues with a low POR threshold voltage because a POR threshold voltage that is too high may result in the POR signal never being triggered or being triggered too late.